An etching method using plasma has been prevailingly applied to a semiconductor device manufacturing step or a LCD substrate manufacturing step. For example, an etching unit comprises an upper electrode and a lower electrode, which are arranged in parallel to each other. Under a condition wherein an object to be processed (for example, a semiconductor wafer) is placed on the lower electrode, plasma is generated from an etching gas by means of discharge between the upper electrode and the lower electrode, so that the semiconductor wafer is etched in accordance with a predetermined pattern.
An instrumental analysis method, such as a mass analysis method or an emission spectral analysis method, is used as a method of detecting an end point of the etching process. Among them, an emission spectral analysis method for the plasma, which is relatively simple and sensitive, is widely used as the end-point detecting method. According to an emission measuring method of the plasma using an emission spectral analysis method, a particular activated species capable of most easily monitored is selected from radicals and ions of the etching gas, decomposition products thereof, reaction products thereof, and so on. Then, an emission intensity of a specific wavelength corresponding to the selected activated species is measured. For example, when a silicon oxide film is etched by using a CF-series etching gas such as CF4, an emission intensity of a specific wavelength of CO* that is the reaction product (483.5 nm or the like) is detected. When a silicon nitride film is etched by using a CF-series etching gas such as CF4, an emission intensity of a specific wavelength of N* that is the reaction product (674 nm or the like) is detected. Then, the end point is detected based on one or more changing points of the detected intensity. That is, according to the conventional end-point detecting method, the wavelength used in the end-point detecting method may be different depending on each etching process.
As described above, according to the conventional end-point detecting method, only a specific wavelength among the emission spectrum of the plasma is monitored to detect the end point. Thus, it is difficult to detect the end point when the change of the emission intensity of the specific wavelength is so small that it is difficult to detect a sufficient change of the emission intensity, for example when an etching process is conducted under a small aperture condition.
U.S. Pat. No. 5,288,367 discloses a method of detecting an end point, wherein a specific wavelength of the emission spectrum is automatically determined by means of a principal component analysis and then the end point is detected based on the specific wavelength. According to the method, the specific wavelength to be used is automatically determined. However, the point that a specific wavelength is used to detect an end point is not different from the conventional end-point detecting method.
The applicant has proposed another method of detecting an end point, wherein a first principal component score about emission intensities of component wavelengths of the plasma is obtained by means of a principal component analysis and then the end point is detected based on a change with time of the first principal component score (Japanese Patent Laid-Open Publication No. 2000-331985). For example, FIG. 9 is a graph showing a change with time of a first principal component score of a component wavelength in a wavelength region of 210 to 410 nm. In the case, a change from a pre-etching state to a post-etching (over-etching) state, that is, an end point of the etching process can be clearly judged. However, if a resist with a much low aperture ratio (for example of 0.25%) is used for the same wafer, a change with time of the above first principal component score becomes as shown in FIG. 10. In the case, the noise is so much that it is difficult to detect the end point.